It is generally known to provide a substrate, such as a medical device or parts of such a device with bio-active coatings for the purpose of enhancing the bio-compatibility of the device when it is introduced into a mammal, such as a human body. Furthermore, it is generally known that various additives, such as therapeutic agents, can be introduced into as substrate for the purpose of releasing such additives into a mammal, such as a human body.
Endoprostheses used for minimally invasive procedures in body conduits, such as, for example, in blood vessels may be provided with bio-active coatings and may also be provided with a means to releasably incorporate a therapeutic agent. Vascular grafts, stents and graft-stent combinations are specific examples of such endoprostheses. Other useful devices include catheters, guide wires, trocars, introducer sheaths and the like.
A therapeutic agent can be incorporated into a substrate and the substrate can be implanted to provide localized delivery of the therapeutic agent. For example, U.S. Pat. No. 5,651,986 discloses a method for administering a therapeutic agent to inhibit growth of a solid tumor. The delivery of the therapeutic agent is accomplished by implanting a device proximally to the tumor where the agent is encapsulated in a biocompatible matrix on the surface of or throughout the implant. The agent is typically released by diffusion, degradation of the matrix, or a combination thereof.
Delivery of other therapeutic agents, such as taxol, is also possible. For example, U.S. Pat. No. 5,733,925 discloses a delivery system for administering taxol by encapsulating the taxol in a polymer. To encapsulate the taxol, the polymer is dissolved in a halogenated hydrocarbon solvent. The encapsulated taxol is implanted where it can be dispersed by the above-mentioned techniques.
Delivery systems for therapeutic agents can be incorporated into other, medical devices, such as a stent. International Patent Application No. PCT/US96/02125 discloses biodegradable compositions that can coat a stent and can be impregnated with a therapeutic agent. The compositions are polymers derived from the condensation of alpha hydroxycarboxylic acids and related lactones, e.g., polylactides or polyglycolides. To form the coating with a therapeutic agent, the polymer must be dissolved in a halogenated hydrocarbon solvent and the therapeutic agent is then dispersed into the solvent/polymer mixture.
The use of solvents is not limited devices that are by themselves or are incorporated as, delivery system for therapeutic agents. Medical articles or devices coated with hydrophilic coatings have been described in a number of references, some of which are discussed below. These patents all employ the use of solvents and/or the requirement for high temperature curing.
U.S. Pat. No. 4,119,094 discloses a method of coating a substrate with a polyvinylpyrrolidone-polyurethane interpolymer. In this method, a polyisocyanate and a polyurethane in a first solvent, such as, methyl ethyl ketone are applied to a substrate. The first solvent is then evaporated and polyvinylpyrrolidone in a second solvent is applied to the treated substrate. The second solvent is then evaporated.
International Patent Applications Nos. PCT/EP92/00918, PCT/EP92/00919 and PCT/DK92/00132 disclose methods for providing medical devices having polyurethane surfaces with a hydrophilic coating of poly(meth)acrylamide. Before application of the hydrophilic coating to the poly(meth)acrylamide substrate surface, it is treated with a compound having functional groups capable of reacting with the polyurethane and the poly(meth)acrylamide, respectively. This compound is typically a di- or higher isocyanate functionality in an organic solvent.
U.S. Pat. No. 5,272,012 discloses a method for applying a protective, lubricious coating to a surface of a substrate. The coating described by the '012 patent includes aprotective compound, such as a urethane; a slip additive, such as a siloxane; and an optional crosslinking agent, such as a polyfunctional aziridine. The surface of a substrate coated with such a composition, however, is not continuously lubricious. Such a coating contains separate physical domains of lubriciousness interspersed within a protective matrix, rather than a continuous layer of a lubricious agent.
U.S. Pat. No. 5,037,677 discloses a method of interlaminar grafting of continuous, hydrophilic anti-fogging coatings for acrylic intra-ocular lenses. Such a method is accomplished using at least two laminae which are not mutually soluble. For example, the '677 patent describes preparing a solution of a copolymer of ethyl methacrylate, butyl acrylate and hydroxyethyl methacrylate in an ethoxy ethyl acetate organic solvent. To this solution is added a molar excess of polyisocyanate. This solution is applied to a plexiglass substrate which is placed in a vacuum oven, where a prepolymer is formed from the two solutes while the ethoxyethyl acetate solvent is evaporated. A 0.2% sodium hyaluronate solution is then applied to the surface of the plexiglass.
The plexiglass is then returned to an oven wherein the hydroxyl groups of the Na-hyaluronate react with the isocyanate groups in the prepolymer layer. Coatings formed in such a manner as the '677 patent suffer from the drawback that organic solvents and/or other toxic chemicals are used as carriers which, if not completely removed prior to introduction of the substrate into the body, can deleteriously react in vivo to cause inflammation, blood clotting and other undesirable side effects. Thus, in order to avoid the use of such organic solvents, some non-solvent methods have been developed.
For example, EP Pat. Application Nos. 92100787.8 and EP 0496 305 A2 disclose methods for preparing a shaped medical. article with a lubricous coating. In these methods, a coating composition that includes a blend of polyurethane and polyvinylpyrrolidone is co-extruded with a substrate polymer to produce a shaped article having on a surface thereof a layer of the coating composition which becomes lubricous when contacted with water.
U.S. Pat. No. 5,041,100 discloses a method for coating a substrate with a mixture of poly(ethylene oxide) and an aqueous dispersion of a structural plastic material, e.g. polyurethane. As an example, this patent discloses a non-crosslinked admixture of poly(ethylene oxide) and a structural plastic material. This composition provides a hydrophilic character to the substrate which may leach to the surface thereof, or be entrapped adjacent to the surface to provide a hydrophilic, reduced friction character thereto, particularly when hydrated.
The methods in the above-described references suffer from the drawback that inter-polymer networks which physically attach hydrophilic polymers to their substrates often break down upon prolonged turbulent flow or soaking. Furthermore, the hydrophilic species are weakly attached to their substrates and can be easily washed away, thereby rendering the underlying article insufficiently lubricous.
International Pat. Application No. PCT/DK91/00163, co-owned with the present invention, discloses a method of providing a medical instrument with a hydrophilic, low-friction coating. This method includes the steps of (1) forming an inner layer on the substrate from an aqueous polymer emulsion, (2) forming an outer layer on top of the inner layer from an aqueous solution of a water-soluble hydrophilic polymer and (3) curing the two layers simultaneously by heating to a temperature above 100.degree. C.
Although the use of organic solvents is eliminated in this method, high curing temperatures must be applied to bond the inner layer to the outer layer. These high curing temperatures are not useful on heat-sensitive materials, as well as, heat-sensitive biomolecules. Thus, heat-sensitive substrates, such as poly(ethylene terephthalate) (PET) balloon catheters cannot be used with this material. Moreover, molecules such as nucleic acids, proteins, peptides, hormones, heparin and the like are heat-sensitive biomolecules which cannot be exposed to such high temperatures without losing their activity.
The art is not limited, however, to medical devices having lubricious coatings disposed on a surface thereof. Rather, medical articles or devices coated with bio-compatible or bio-active agents have also been described, some of which are set forth below. All of these patents employ various inefficient and/or harsh methods for attaching the bio-compatible/bio-active agent to the surface of a medical article.
For example, U. S. Pat. No. 5,541,167 describes a thrombo-resistant and defoaming coating for blood contacting surfaces including bubble oxygenators, blood filters, etc. This coating includes a commercial preparation of polydimethylsiloxane and silicon dioxide and a quaternary ammonium complex of heparin, such as stearyldimethylbenzyl. This coating, however, suffers from the drawback that the defoaming and heparin components are dissolved in an organic solvent, such as methylene chloride. Such solvents can denature and reduce the bio-activity of bio-active agent, such as heparin. Furthermore, such organic solvent systems produce environmentally hazardous waste, as well as attacking certain polymer substrates.
In a different approach to rendering an implantable medical device bio-compatible, U.S. Pat. No. 5,360,397 describes a porous bio-compatible and bio-stable polyurethane mesh for a catheter made from polycarbonate urethane. This mesh is sputter coated and/or impregnated with a bio-active agent, such as for example, a bactericide. A catheter treated in such a manner, however, is imparted with transient bio-activity at best because the bio-active agent is not covalently bound to the surface thereof. Furthermore, the process of making such a catheter is inefficient because the porous polyurethane mesh must be attached to the surface of the catheter prior to the application of the bio-active agent.
Still further, U.S. Pat. No. 5,263,992 describes a medical device having a bio-compatible coating which includes a bio-compatible agent, such as for example, heparin or streptokinase and a chemical linking moiety. This chemical linking moiety has a structure represented b: A-X-B, wherein A is a photochemically reactive group, B is a reactive group which responds to a different stimulus than A and X is a non-interfering skeletal moiety, such as a C.sub.1 -C.sub.10 alkyl. The bio-compatible agent is covalently linked to the surface of the medical device via the linking moiety. In particular, the photochemically reactive group (A) when activated covalently binds to the surface of the medical device. The remaining unreacted reactive group (B) when activated covalently binds to the bio-compatible agent and anchors it to the surface of the medical device. Such devices, however, are difficult and inefficient to produce because they require the use of two separate stimuli to activate the A and B groups of the chemical linking moiety, respectively. Furthermore, the UV light used to activate the A group of the chemical linking moiety for covalently binding it to the surface of a medical device can denature bio-active agents. Such denaturization reduces the bio-activity of such agents and can result in undesirable medical outcomes, such as, clot formation in the case of an anti-thrombogenic agent.
The present invention, however, is directed to hybrid coatings for substrates, particularly medical devices, that function as a bio-compatible surface, as well as a drug delivery vehicle. These coatings are particularly advantageous because they can be applied to devices which are sensitive to high processing temperatures, such as (PET) balloon catheters and other polymeric or heat sensitive materials or biomolecules. Moreover, these coatings also function as therapeutic agent delivery vehicles for delivering such agents to targeted areas in the body. In this regard, the present invention discloses the use of two-component bio-compatible coatings which are both aqueous based. Such coatings are mutually soluble and do not pose the increased medical risks associated with coatings containing organic solvents. Furthermore, preparation of the present aqueous coatings is more efficient because vacuum baking substrates is not required as there are no organic solvents that must be removed. Moreover, because the bio-active surface is covalently bonded to the polymer of the first coating, this coating is permanently attached to the substrate unlike certain of the transient coatings discussed above. Thus, the bio-active surface provides long term bio-compatibility, e.g. thrombo-resistance, while the first coating layer allows a therapeutic agent, i.e. an anti-smooth muscle cell (SMC) proliferation agent to be released which both function to reduce or prevent restenosis of, e.g., an endoprosthesis by minimizing thrombus formation and SMC proliferation.
In summary, the prior art compositions and methods suffer from the drawback that they use organic solvents in their coating layer and/or cure at high temperatures, are transient and/or are inefficient to produce. Thus, there is a need for improved bio-compatible coatings which enhance the bio-compatibility and abrasion-resistance of the surface of heat sensitive medical devices and which also function as therapeutic agent delivery vehicles. The present invention is directed to meeting these and other needs.